Optical moldings are particularly limited by the prior art. The present invention may be used in optical-grade thermoplastic molded products such as edge-gated Rx spectacle lens, plano eyewear, instrument optical lenses & prisms, flat-panel display lenses, information bearing optical data cards, halographic displays, reflective and/or transmissive optics, precision molded plastic mirrors, refractive optical elements with a light bending function using multiple concentrically arrayed facets (such as fresnels) or multiple molded lenslet arrays. Even very large shapes such as fresnel- or mirror-type solar collector panels, or front/rear projection screens, or automotive windows could be uses.
The limitations learned from the below-mentioned patents drove us to look at ways to overcome filling problems. Typical computer simulation software would show filling problems, if the meltflow pathlength was too long, or the aspect ratio is too high. Such product configurations have a large aspect ratio for filling, defined as the length of the meltflow pathlength divided by the cross-sectional thickness. Conventional alternatives all have drawbacks if the mold cavity has just 1 gate, even if centrally located. Substituting lower melt viscosity resin with less molecular weight or less reinforcement gives poorer properties to the molded part. Substituting thicker cross-sectioned part gives slower cycle time & higher material costs to the molded part. lower melt viscosity resin or thicker molecular weight. Thus, in search of a way to get shorter meltflow pathlength and lower aspect ratios, multiple gates can be spaced out along the perimeter of the mold cavity. However, when the resulting multiple meltfronts converge and intersect, cosmetically unacceptable knitlines (visible surface flaws) and weldlines (internal weak-spots of poor mechanical strength are not created.
One approach to this problem is multi-gated sequential filling of injection molds for large-surface-area parts and/or thinwalled parts. Flow lengths are now shortened, but the knitlines can be avoided by opening one valve gate at a time. In this sequential fill, the meltfront from the first-to-be-opened gate must pass by the location of the second-to-be-opened gate before this second gate is opened, and so on. As each subsequent gate opens, its melt blends into melt from the previously-opened gates to ideally provide a single smoothly-flowing meltfront driven by multiple short-flowpath gates. No gate is opened to injection before the single smoothly-flowing meltfront has swept by, thus avoiding multiple meltfronts converging and intersecting in knitlines. A recent example of multi-gated sequential filling in Betters et al (U.S. Pat. No. 5,762,855 issued Jun. 9, 1998). It uses mechanically closed valve gates to keep the melt within the next shot at desirably high pressurizations, to avoid splay and other surface defects. It mentions briefly . . . "improved knitline appearance" . . . on column 2, line 18 without further elaboration or support. Another example of multi-gated sequential filling is Hunerberg et al (U.S. Pat. No. 5,135,703 issued Aug. 4, 1992), which also comprises gas injection behind the moving meltfront. Multi-gated sequential filling is reportedly successful in thinwalling opaque electronic housings (i.e. cellphones and laptops) and hard-to-fill large opaque automotive moldings (i.e. bumpers and body panels), but no known optical lenses use it.
A different prior art approach teaches to allow knitlines and weldlines to form, but then to use a plurality of substantially opposing gates to alternately pressurize and depressurize the melt, in coordination with ohe another. So, when one gate is acting to pressurize against the melt, its opposing gate is depressurizing. Then in accordance with a programmed control, they switch roles. And so on, such that at the original intersection of the 2 opposing meltfronts, shearing forces may cause molecular entanglements while the melt is still mobile. Such reciprocating "push-pull" forces are believed to strengthen the weldlines (internal weak-spots of poor mechanical strength) and improved fiber reinforcement orientation. One of the better-known such "multi live feed" approaches is offered by Cinpres, (Allan et al, U.S. Pat. No. 4,925,161 issued May 15, 1990, U.S. Pat. No. 5,156,858 issued Oct. 20, 1992 and U.S. Pat. No. 5,160,466 issued Nov. 3, 1992), in its "Scorim" process available for licensing. Similar in effect but requiring 2 separate injection barrels to implement in Klockner's approach, per Gutjahr et al (U.S. Pat. No. 4,994,220 issued Feb. 19, 1991), dealing with improved orientation of liquid chrystal polymers. Also similar in effect but perhaps with simpler hardware (requires only injection barrel to implement, and capable of running multiple mold cavities) is Husky's approach, per Arnott (U.S. Pat. No. 5,069,840 method patent issued Dec. 3, 1991; U.S. Pat. No. 5,192,555 apparatus patent issued Mar. 9, 1993). Thermold employs an accumulator between plastication and mold, in Ibar (U.S. Pat. No. 5,605,707 issued Feb. 25, 1997). Other newer ones attempting to be similar in effect by trying to act locally upon just the weldline include Groleau (U.S. Pat. No. 5,766,654 issued Jun. 16, 1998), and Gardner et al (U.S. Pat. No. 5,538,413 issued Jul. 23, 1996) both employing reciprocating "packing pins" located beneath the weldline to pulse, and Furugohri et al (U.S. Pat. No. 5,225,136 issued Jul. 6, 1993), employing a "well" as a controllable reservoir for molten resin, located between the gate and the weldline, to . . . "cause migration of the resin at the weld". However, it is believed that none of these "multi live feed" approaches are successfully employed to the optical lens molder's problems of how to eliminate cosmetically unacceptable knitlines (surface flaws) on the usable portion of transparent amorphous thermoplastic molded lenses. Inasmuch as each of these "multi live feed" approaches still predicates that the multiple meltfronts converge and intersect, they can only remedy the weldlines (internal weak-spots of poor mechanical strength) after they are now created.
All of the above-mentioned multi-gated sequential filling and "multi live feed" approaches are still running substantially isothermally, with respect to the measured temperature of the injection mold cavity blocks and circulating coolant therein. That is to say, those metal mold temperatures and coolant temperatures are always set well below the glass transition temperature Tg of the amorphous thermoplastic throughout the whole injection molding cycle.
Such is also true of the only prior art reference known to Applicants, wherein at least 2 opposing gates are employed to feed a transparent amorphous thermoplastic melt into a single mold cavity, to improve filling and packing of an optically-functioning molding. Kanewske III et al (U.S. Pat. No. 5,376,313 issued Dec. 27, 1994) is molding a lowcost disposible testtube-shaped plastic assay cuvette from . . . "acrylic, polystyrene, styrene-acrylontrile, polycarbonate . . . " (col. 9, ln 51-2) at . . . "temperature of the mold cavity 508 is preferably from between about 100 F. and about 140 F.; and the temperature of the mold core 504 is preferably from between about 60 F. and about 100 F." . . . (col. 9, ln 63-67), all of which are very far below the Tg of any of their resins mentioned. In order to get the desired low levels of molded-in stresses in the "optical read region" (through which absorbance of a known light beam is measured), this patent says that it was necessary to locate the gate at least some minimum distance away from "optical read region", and since this reduces ease of filling, the preferred embodiment employs at least 2 such gates at substantially opposing locations with substantially simultaneous injection into both (presumably, to maintain symmetry). This patent is silent on the presence of a knitline, but those skilled in the art would expect a very long knitline.
Let us now move away from these prior art approaches which are still running substantially isothermally. Various "non-isothermal" prior art approaches have already been previously cited by Applicants in our U.S. Pat. No. 5,376,317 (Maus et al) issued Dec. 27, 1994, incorporated herein by reference. In the USPTO examination of U.S. Pat. No. 5,376,317, the closest prior art was determined to be Muller (U.S. Pat. No. 4,963,312 (method) issued Oct. 16, 1990 and U.S. Pat. No. 5,055,025 issued Oct. 8, 1991 (apparatus)). Muller is drawn to thinwall packaging molding, which has somewhat different problems than Applicants' focus on optical lens and quality of microstructured replicated surfaces. Like Applicants, Muller heats the mold surface before injection, so as to overcome the melt's resistance to flow and to improve filling of its difficult product configurations. Unlike Applicants, Muller apparently constructed his mold cavity members without regard to their heat transfer rates (he is silent on his materials of construction; we assume tool steels), so to achieve fast cycle times, he chose to make his mold cavity members very thin, which then required him to stop flowing his circulating heat carrier and hold it at maximum static pressure . . . "during injection of plastic . . . for supporting the thinwalled members" (see his Abstract) against unwanted mechanical deflection. Applicants solved this problem by our choice of materials of construction and sufficient thicknesses of same for proper loadbearing, so our heat transfer fluid can continue flowing through the mold during filling and packing phases of the molding cycle.
In addition to those cited "non-isothermal" prior art approaches, there are a couple newer ones which need to be commented on.
Yamaguchi et al (U.S. Pat. No. 5,399,303 issued Mar. 21, 1995) is drawn to a similar field--optical lenses--as Applicants, but with a very different sequence of method steps . . . "filling resin in a metal mold held at a temperature lower than a glass transition temperature of the resin, then pressurizing the resin under comparatively high pressure so as to expedite hardening of the resin by raising its glass transition temperature, then reducing the pressurization of the resin to comparatively low pressure, and increasing, generally in association, the temperature of the mold cavity surface higher than the glass transition temperature of the resin so as to form a molten layer on a front face of the resin surface, and finally raising the pressurization of the resin to a medium pressure and lowering, generally in association, the cavity surface temperature of the metal mold so a to reduce the temperature for the withdrawal of the product" . . . (see Yamaguchi's Abstract). In this sequence of steps, Yamaguchi resembles the Uehara (U.S. Pat. No. 5,093,049 issued March 1992) cited in our U.S. Pat. No. 5,376,317. By this sequence of steps, Yamaguchi does nothing to prevent the formation of knitlines or weldlines, were he to have been running myopic-prescription polycarbonate Rx spectacle lenses having thin centers and thick edges. Once the knitline is formed on the molded lens surface, merely re-heating per Yamaguchi . . . "so as to form a molten layer on a front face of the resin surface" . . . will not be adequate to remove the objectionable knitline. Nor does Yamaguchi teach the benefits of reducing geometric resistance to filling (like Applicants' use of variable volume mold cavity with injection compression sequence), nor reducing meltflow pathlength (like Applicants' use of opposing gates).
Byon (U.S. Pat. No. 5,762,972 issued Jun. 9, 1998) is not drawn to a similar field--optical lenses--as Applicants, but rather general problems of filling. Byon heats the mold before injection (by induction) to improve the fluidity of the resin melt, so there is no need to increase injection pressure. He does so by induction because it is faster than by the heat pipes employed in his cited prior art. Only very briefly is there any mention of weldlines, and no mention of knitlines . . . the fluidity of the resin fluid filling up the cavity is increased. In addition to these, the luster of the product is enhanced while reducing flow marks, weld lines, etc." (col. 4, ln 22-24). Byon is silent on the criticality of heating the mold to at least the Tg. Apparently, he is satisfied by . . . "The rise in mold temperature inhibits the cooling of the flowing resin fluid to maintain the fluidity of the resin fluid, so that there is no need to increase the injection pressure" . . . (col. 2, ln 1-3). Also, Byon does not teach the benefits of reducing geometric resistance to filling (like Applicants' use of variable volume mold cavity with injection compression sequence), nor reducing meltflow pathlength (like Applicants' use of opposing gates).